The way it does this is ridiculous: instead of calling a simple
GET request to save the page, since an entire page’s contents
would quickly overrun any URL-length limits in the stack, it
injects a FORM with a POST action and populates a hidden value
with the page contents.

But form-data requests from browsers aren’t Gzip-compressed, so
the resulting data is huge and needs to be sent over people’s
(often slow, often mobile) upstream connections. So I found an
open-source DEFLATE implementation in Javascript — really —
and the bookmarklet compresses the page data right there in the
browser before sending it.